In the field of LED drivers for offline applications, solutions are demanded to drive the LEDs over a large power range with a high reliability, in particular to drive the LEDs at very low power precisely and having a low sensitivity to external interferences and noise.
In the field of LED drivers LLC converters are commonly known for driving LEDs. The LLC converter controls the output power provided to the load by switching two controllable switches and by providing an alternating input voltage to an electromagnetic transformer. The energy transferred by the LLC converter is related to the energy change in a capacitor between the two switching states. The energy provided to the load is controlled by switching the controllable switches.
U.S. 2011/0164437 A1 discloses an LLC converter, wherein the output power is controlled by varying the duty cycle of the controllable switches. Alternatively, the output power of the LLC converters is controlled by changing the switching frequency of the controllable switches as disclosed by U.S. Pat. No. 7,313,004 B1.
A further method to control the LLC converter is to trigger the switching actions of the controllable switches on the basis of the transformer voltage. The switching actions are triggered when the transformer voltage exceeds a threshold voltage and causing the switches to reverse when the transformer voltage drops below a second threshold voltage. The output power provided by the threshold-controlled LLC converter can be controlled by changing the input voltage provided to the controllable switches and by changing the threshold voltages. The change of the input voltage results in a change of the frequency of the controllable switches and adjusting the threshold voltages linearly with the change in the input voltage may result in an approximately constant output power. The voltage-threshold control of the LLC converters allows a better linearity than a frequency control and provides a constant output power for powering the load.
The disadvantage of the threshold-controlled LLC converters is that they are complicated and do not provide a robust low power operation. Since the threshold levels are set above the input voltage for providing low power to the load, the transformer voltage exceeds the threshold for a very short time resulting in a high switching frequency of the controllable switches. Noise or a variation of the threshold level can change the switching points of the controllable switches slightly such that no power will be delivered to the load which may cause random disruption of the output power. The disruption caused by the noise can also result in an oscillation causing an inconsistent output power. Further, the known LLC converter provide a reduced output control linearity.